Granular-media filters have been used extensively to remove suspended solids from water in water treatment plants. Such filters also are finding widespread application for removing suspended solids from wastewater streams which have had various degrees of pretreatment. The filtered solids are trapped at the surface of the filter bed and in the interstitial spaces within the filter bed. Granular-media filters may be designed and operated with flow in the upward or downward direction, although downflow filters are the more common arrangement.
Granular-media filters are cleaned by a periodic washing operation (so-called "backwashing" for downflow filters) in which water is passed upwardly through the filter bed. The rate of water applied for washing depends on the size of the granular-media and the desired fluidization or bed expansion characteristics. The filters can be cleaned more completely and more efficiently in terms of the completeness of solids removal and the amount of water needed for the washing operation by agitating the bed with air or other means simultaneously with the upward flow of backwash water. However, because of the extreme turbulence caused by this agitation, many particles of media are carried over the discharge edge of the backwash trough and consequently are lost from the filter. Normally there is no way to recover this media.
It has been proposed to employ baffles around the backwash trough or launder to reduce turbulence in the vicinity of the overflow weir, thereby permitting the granular-media carried up with the wash water to settle, or at least not to be carried into the wash trough, and lost from further use in the filter bed. One such device is shown in the United States patent of Row et al No. 2,453,345. As shown in the patent, it consists of a vertical baffle on each side of the launder trough, which define flow channels of uniform cross-section. The trough or launder has a rectangular cross-section. Additional baffles are mounted at the lower corners of the trough to extend outwardly and downwardly beneath the lower ends of the vertical baffles, defining a restricted inlet to the outflow channels. The liquid flow velocity through the outflow channels is controlled so that it does not exceed the settling velocity of the filter bed material. However, it is stated that any filter bed material carried up by the wash liquid is generally carried to a region outside of the flow channels defined by the baffles. It is claimed that loss of filter bed material during washing is substantially eliminated and that washing time and washing fluids are considerably reduced. The launder trough and baffle assembly is said to function equally as well whether a wash liquid is alone used or the wash is a mixture of water and air.